The present invention relates to a process for upgrading mixtures of hydrochloric and hydrofluoric acid. The invention is more particularly directed towards a process for upgrading hydrofluoric acid. It is also directed towards a process for manufacturing gaseous boron trifluoride.
Chlorofluorohydrocarbons (for example chlorodifluoromethane), hydrofluorocarbons (for example tetrafluoroethane) or hydrofluoroolefins (for example tetrafluoropropene) are generally obtained by reacting a chlorinated hydrocarbon with hydrofluoric acid in liquid or gas phase in the presence of a catalyst. Hydrofluoric acid is generally used in super-stoichiometric amount in this reaction. Besides the desired fluoro compound, hydrochloric acid is formed, which, in the presence of hydrofluoric acid, poses a separation problem.
The recovery of hydrochloric acid and hydrofluoric acid in the abovementioned reaction is not easy. Standard separation methods, for example distillation, do not make it possible to obtain hydrofluoric acid of sufficient purity. It should be noted that residual amounts of chlorides, greater than 50 mg/dm3, in hydrofluoric acid are harmful in certain applications such as the stripping of steel plates.
To improve the separation of hydrochloric acid from hydrofluoric acid and soluble fluorides, document U.S. Pat. No. 2,343,462 proposes to distill an aqueous solution containing hydrochloric acid and hydrofluoric acid or soluble fluorides in the presence of boric acid or a soluble borate. The examples illustrate the purification of hydrochloric acid contaminated with hydrofluoric acid or fluorides using boric acid. However, this separation method generates fluoroboron mixtures contaminated with chlorine, which are thus difficult to upgrade.
The term “fluoroboron mixtures” means dilute solutions of boron trifluoride hydrates composed of mixtures of BF3.2H2O, boric acid H3BO3 and hydrated fluoroboric acids HBF4.zH2O as explained in the scientific publications by Christian A. Wamser, J. Amer. Chem. Soc. Volume 73, pages 409 to 413 of 1951 and of the supplement Von Stephan Pawlenko, Zeitschrift für anorganische and allgermeine Chemie, Band 300; 1959, z not being a figure determined with precision.
The term “hydrochlorofluoric acid” means a mixture of HF, HCl and H2O, optionally with very small amounts of metal chloride.
Moreover, boron trifluoride (BF3) is a gas that is mainly used in the industry as a catalyst in a large number of reactions: polymerization, esterification, alkylation, isomerization. Gaseous, anhydrous boron trifluoride is generally obtained by reacting liquid anhydrous hydrofluoric acid with boric acid dissolved in sulfuric acid and by supplementary addition of oleum. Gaseous boron trifluoride is also obtained by treatment with sulfuric acid or sulfuric acid as a mixture with oleum of a concentrated aqueous solution of BF3 hydrate. However, the presence of impurities, such as hydrogen chloride (due to the generation of HCl gas at the same time as the BF3 gas) in the concentrated aqueous solution of BF3 hydrate poses a problem during the treatment with sulfuric acid. Specifically, the separation of gaseous BF3 and of gaseous HCl is not only very difficult, but also very expensive.